Lithography systems and tools are used to print features in a variety of manufacturing applications. Photolithography systems use a mask or reticle to expose features onto an object. In semiconductor manufacturing, for example, a reticle is exposed by an exposure beam. An optical system then projects a reduced image of the reticle onto a silicon wafer. In this way, circuit features can be printed on a semiconductor substrate.
Maskless lithography systems have been developed, which do not require use of a mask or reticle. Current maskless lithography mask writing systems project a pattern to be printed onto a moving object. For example, a pattern of circuit features can be projected onto a moving wafer. In one example, a silicon wafer can be coated with a photoresist. The pattern is projected in a sequence of exposures (also called shots). Each shot projects an image of a pattern from one or more spatial light modulator (SLM) arrays.
An SLM array is a programmable array of elements that modulates the light projected onto the object. One type of SLM is a digital micromirror device (DMD). A DMD is often used in a reflective mode. Each mirror within the DMD can be programmed to reflect light such that it passes in or out of an optical path. A DMD then acts as a binary switch that outputs light in one of two binary states: “on” or “off.” Shading or grayscale variation of the light intensity can be achieved by changing the duty cycle of a laser pulse source so as to increase or decrease the exposure time. In this way, the DMD can be programmed to project a desired pattern onto an object such as a wafer by controlling the individual micromirror elements to reflect in a desired pattern.
Another type of SLM is a transmissive liquid crystal light valve (LCLV). An LCLV is typically arranged in a transmissive configuration. Like the DMD, the LCLV is programmed such that the individual light valves are controlled to project a desired pattern onto an object. Typically, polarized light is passed through the LCLV. The individual valves are controlled such that a polarization state is rotated, thereby modulating the intensity of the polarized light that passes through the respective valve. The polarization of an individual light crystal valve can be controlled to pass light in a binary fashion (on/off) or at different intensity levels with shading or grayscale variation by adjusting the rotation of polarization.
Maskless lithographic tools and techniques are increasingly called upon to print patterns at high resolution. For example, in the manufacture of semiconductor dies or chips, patterns of circuit features, such as lines for passive or active devices, often need to be printed at a high resolution to improve the packing density of circuit elements and reduce the pitch of the pattern. At high resolution, the alignment of the SLM arrays relative to the wafer becomes even more important. In order to obtain a desired distribution of a dose in the resist with high accuracy, SLM arrays need to be aligned with respect to each other and with respect to the object being printed.
Alignment needs to be precise. Each SLM array used to dynamically generate a pattern in maskless lithography needs to be properly aligned to other SLM arrays and to an absolute coordinate system such as a general frame of reference in an object plane. This alignment needs to be performed to a small fraction of the size of a single pixel in directions parallel to the object plane and to a small fraction of a wavelength in a direction normal to the object plane.
Two types of misalignment can occur: static and dynamic. A static misalignment or a static deformation refers to a residual misalignment or deformation of an SLM array compared to a desired, aligned position. A static misalignment or deformation generally does not change during a scan or shot. A dynamic misalignment or deformation refers to a time-varying displacement or deformation. Dynamic misalignment and/or deformation can occur during printing when heat and vibrations produced by the operation of each SLM array and supporting electronics can cause SLM arrays to dynamically displace as a rigid body and deform (i.e., warp). Both static and dynamic misalignments and deformations (if left uncompensated) degrade the accuracy of the printed pattern. Deformations and misalignments in the directions parallel to the object plane introduce spatial distortions in the image. Deformations and misalignments in the direction normal to the object plane result in defocus and/or telecentricity effects.
What is needed is a system and method for compensating each SLM array's static and dynamic spatial misalignment and deformation.